[PATCH] Corrections for C++2a std::unwrap_reference traits

[Jonathan Wakely]

The P0318R1 paper added to the C++2a draft recently was not the latest
version of the paper, and should have included these changes. These
changes will be made to the working draft via a Defect Report, so I'm
applying them to libstdc++ now.

        * include/std/type_traits (unwrap_reference_t): Define for C++2a.
        (unwrap_ref_decay): Remove inheritance from unwrap_reference.
        * testsuite/20_util/unwrap_reference/1.cc: Adjust test to use alias.

Tested powerpc64le-linux, committed to trunk.



commit 9e09c9a4b99b3be725144aeaebe902783a9e70a3
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Tue Apr 23 10:58:22 2019 +0100

    Corrections for C++2a std::unwrap_reference traits
    
    The P0318R1 paper added to the C++2a draft recently was not the latest
    version of the paper, and should have included these changes. These
    changes will be made to the working draft via a Defect Report, so I'm
    applying them to libstdc++ now.
    
            * include/std/type_traits (unwrap_reference_t): Define for C++2a.
            (unwrap_ref_decay): Remove inheritance from unwrap_reference.
            * testsuite/20_util/unwrap_reference/1.cc: Adjust test to use alias.

diff --git a/libstdc++-v3/include/std/type_traits 
b/libstdc++-v3/include/std/type_traits
index c3cb67a457d..e0ac5c88dfc 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -3067,9 +3067,12 @@ template <typename _From, typename _To>
   template<typename _Tp>
     struct unwrap_reference<reference_wrapper<_Tp>> { using type = _Tp&; };
 
+  template<typename _Tp>
+    using unwrap_reference_t = typename unwrap_reference<_Tp>::type;
+
   /// Decay type and if it's a reference_wrapper, unwrap it
   template<typename _Tp>
-    struct unwrap_ref_decay : unwrap_reference<decay_t<_Tp>> { };
+    struct unwrap_ref_decay { using type = unwrap_reference_t<decay_t<_Tp>>; };
 
   template<typename _Tp>
     using unwrap_ref_decay_t = typename unwrap_ref_decay<_Tp>::type;
diff --git a/libstdc++-v3/testsuite/20_util/unwrap_reference/1.cc 
b/libstdc++-v3/testsuite/20_util/unwrap_reference/1.cc
index 38feca4bb5b..243526725a0 100644
--- a/libstdc++-v3/testsuite/20_util/unwrap_reference/1.cc
+++ b/libstdc++-v3/testsuite/20_util/unwrap_reference/1.cc
@@ -28,7 +28,7 @@ template<typename T, typename U = T>
   {
     using std::unwrap_reference;
     using T2 = typename unwrap_reference<T>::type;
-    static_assert(expect_same<T2, typename unwrap_reference<T2>::type>::value);
+    static_assert(expect_same<T2, std::unwrap_reference_t<T2>>::value);
     return expect_same<T2, U>::value;
   }